Nondestructive flaw detection
2016-11-11 Rongda test 237 times
Detection Purpose: Determination of pearlite content in metal
Detection range: Oil pipeline, boiler, liquefied gas tank and other pressure vessels, metal and other products
The eutectoid steel is heated and fully austenitized, then cooled slowly, and eutectoid decomposition occurs at a temperature slightly lower than A1. The product is a mixture of & alpha; and Fe3C. It is called pearlite because of its image like mother of Pearl under the microscope. Pearlite is the mechanical mixture of ferrite and cementite. It is the eutectoid transformation product of steel. Its morphology is that ferrite and cementite alternate with each other, such as fingerprints, arranged in layers. According to the distribution of carbide, it can be divided into lamellar pearlite and spherical pearlite. The morphology of the pearlite is a kind of layered complex, which is alternately overlapped by ferrite thin layer and cementite thin layer. Lamellar pearlite can be divided into three types: coarse lamellar, medium lamellar and fine lamellar (pearlite, sorbite and troostite). 1) the distance between pearlite sheets depends on the undercooling of austenite during decomposition. The greater the undercooling, the smaller the distance between the pearlite sheets. 2) the pearlite lamellae formed at A1 ~ 650 ℃ is relatively thick, and the parallel wide strip ferrite and fine strip cementite can be distinguished at a magnification of more than 400 times under the metallographic microscope, which are called coarse pearlite and lamellar pearlite, or pearlite for short. 3) the pearlite formed at 650 ~ 600 ℃ is magnified 500 times by metallographic microscope. Only one black line can be seen on the cementite of pearlite, and the lamellae can be distinguished only by magnified 1000 times. It is called sorbite. 4) the pearlite formed at 600 ~ 550 ℃ is magnified 500 times by metallographic microscope, and the pearlite lamellae can not be distinguished. Only the black globular structure can be seen, and the lamellae that can be distinguished only by magnified 10000 times by electron microscope is called troostite. The structure of granular cementite dispersed on ferrite matrix is called "granular pearlite" or "spherical pearlite", which is generally obtained by spheroidizing annealing or quenching and tempering at medium and high temperature.
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